Heat exchanger for a co2 vehicle air conditioner

ABSTRACT

A heat exchanger is described, having a first channel through which a stream of refrigerant flows on the high pressure side in a first direction, and a second channel, separate from the first channel, through which refrigerant flows on the low pressure side in a second direction opposite the first direction, wherein the first and second channels each have a large number of small channels ( 11, 12, 13 , . . . ) formed in or on individual heat transfer plates ( 1, 2, 3 , . . . ), and a plurality of layers of the heat transfer plates are soldered or welded together.

FIELD OF THE INVENTION

[0001] The present invention relates to a heat exchanger having a first channel through which a stream of refrigerant flows on the high pressure side and a second channel, separate from the first channel, through which refrigerant flows on the low pressure side.

BACKGROUND INFORMATION

[0002] A heat exchanger of this sort is known in one application as an inner heat exchanger of a CO₂ vehicle air conditioner from Status Report No. 20 of the Deutscher Kälte-und Klimatechnischer Verein [German Refrigeration and Air Conditioning Association]: “Kohlendioxid—Besonderheiten und Einsatzchancen als Kältemittel” [Carbon dioxide-characteristics and prospects for use as a refrigerant], page 137 (November 1998).

[0003] The interest in natural refrigerants as an alternative to CFC is increasing, as an outgrowth of the rules and regulations for eliminating the use of CFC.

[0004] The range of natural refrigerants includes carbon dioxide, which is non-combustible and nontoxic. Research on carbon dioxide, which was first used as a refrigerant in 1866 but disappeared from use in the nineteen-fifties, experienced a renaissance in the eighties through the work of Lorentzen and his associates. Future fields of use are vehicle air conditioning, heat pumps, transportable air conditioners of small capacity, air dehumidifiers, and dryers.

[0005] To increase the performance and efficiency of the CO₂ process, an inner or internal heat exchanger has been proposed. The internal heat exchanger has refrigerant (CO₂) flowing through it. First, it flows through on the way from the gas cooler to the evaporator, and the second time between the evaporator and the compressor. The main task of the internal heat exchanger is to enable additional cooling by the internal heat exchanger at times when ambient temperatures are high, where the gas cooler consequently is not able to cool the refrigerant sufficiently prior to expansion. The flow of heat passes from the high pressure side, downstream from the gas cooler, to the low pressure side, downstream from the evaporator (before entering the compressor). The refrigerant, still partially liquid on the suction side, then vaporizes completely before reaching the compressor. The internal heat exchanger is practically designed as a counterflow heat exchanger.

[0006] The internal heat exchanger known from the status report of the Deutscher kälte-und klimatechnischer Verein mentioned above is manufactured at present for example as a counterflow double-pipe heat exchanger. The pipe profile is made of extruded aluminum. On the high pressure side the refrigerant flows through the inner pipe, for reasons of strength. A difficulty here is the dimensioning of the heat transfer surface or cross-sectional area of flow on the suction side in order to achieve a satisfactory heat transfer coefficient together with an acceptable drop in pressure of the refrigerant.

SUMMARY OF THE INVENTION

[0007] An object of the present invention is to specify a small, compact heat exchanger in which a very large heat transferring area is realizable in a small volume, which is suitable for use as an inner heat exchanger in a CO₂ air conditioner.

[0008] Because according to a significant aspect of the present invention the first and second channels are each formed from a large number of small channels located in or on individual heat transfer plates, and because a plurality of layers of the heat transfer plates are joined together, for example by soldering or welding, a heat exchanger of this sort may be made very compact, i.e., having a very small volume, and at the same time with a large heat transferring area. Because of the large number of small channels and the design and manner of operation of the heat exchanger using the counterflow principle, it is possible to improve the transfer of heat compared to the known implementation while keeping the pressure drop at an acceptable level.

[0009] Because of the large number of small channels, the heat transferring area may be increased significantly.

[0010] It is preferable that the hydraulic diameter of the small channels be chosen such that the product of the heat transfer coefficient and the heat transferring area on the high pressure side corresponds to the product of the heat transfer coefficient and the heat transferring area on the low pressure side.

[0011] Alternatively or in addition, the flow path may be chosen, for example by routing the small channels in a zigzag pattern, so that the product of the heat transfer coefficient and the heat transferring area on the high pressure side corresponds to the product of the heat transfer coefficient and the heat transferring area on the low pressure side.

[0012] Because the channels are produced on or in the plates using a manufacturing process that removes or builds up material, the channels—that is, the channel diameters—may be made very small in conformity with the operating pressure conditions.

[0013] Because of its compact design, the proposed heat exchanger may be used for very high pressures up to around 150 bar.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 in a first embodiment shows the structure and flow conditions of a heat exchanger according to the present invention, made up of individual layers of sheet metal.

[0015]FIG. 2 shows a first arrangement of a compact heat exchanger.

[0016]FIG. 3 shows a second arrangement of a compact heat exchanger.

[0017]FIG. 4 shows a third arrangement of a compact heat exchanger.

DETAILED DESCRIPTION

[0018] The embodiment of a heat exchanger according to the present invention depicted in FIG. 1 is very compact because of the fact that individual lamellar heat transfer plates 1, 2, 3, which are soldered or welded together and are packed between two cover plates 8, 9, have small channels 11, 12, 13 and flow orifices 4, 5, 6, 7. High pressure CO₂ which flows in at an inlet orifice 14 of left cover plate 8 (arrow EH) flows through flow orifice 4 of the left heat transfer plate to middle heat transfer plate 2, downward through the latter's channels 12 in the direction of the arrow, and from there again flows to the left through flow orifice 6 of first heat transfer plate 1 and out through outlet orifice 16 of cover plate 8 (arrow AH). In addition, as indicated by the hatched arrows, low pressure CO₂ (arrow EN) flows into an inlet orifice 15 of left cover plate 8, through channels 11 of first heat transfer plate 1 from bottom to top, continuing through flow orifice 5 of second heat transfer plate 2 to third heat transfer plate 3 and there also through the latter's small channels 13 from bottom to top and through the corresponding flow orifices 7 of third, second, and first heat transfer plates 3, 2, and 1 and then out through outlet orifice 17 of left cover plate 8 (arrow AN).

[0019] In this way, the depicted heat exchanger has refrigerant on the high pressure side (black arrows) flowing through it in a first direction, and refrigerant on the low pressure side (hatched arrows) flowing through it in a counterflow.

[0020] The structure of the heat exchanger depicted in the figure, having only three heat transfer plates 1, 2, 3, is of course only an example.

[0021] The heat exchanger shown in FIG. 1 is thus made up of individual layers defined by the heat transfer plates, having CO₂ flowing through them in counterflow on the one side at high pressure (up to nearly 150 bar) and high temperature, and on the other side at low pressure (up to approximately 60 bar) and low temperature.

[0022] To adapt the heat exchanger ideally to the occurring heat transfer conditions, allowance is made for the fact that the heat transfer is determined by the material properties of the fluid and by the flow condition. However, the heat transfer coefficient on the low pressure side is generally significantly smaller than that on the high pressure side. To utilize the volume of the heat exchanger most efficiently, an effort should therefore be made to match the product of the heat transfer coefficient and the heat transferring area on the high pressure side to the product of the heat transfer coefficient and the heat transferring area on the low pressure side. This may be done in the compact heat exchanger shown, which is made up of individual profiles, i.e., heat transfer plates 1, 2, 3, in which small channels 11, 12, 13 are incorporated, by appropriate adjustment of the hydraulic diameter of small channels 11, 12, 13.

[0023] The possibility also exists of increasing the heat transferring area, i.e., the heat transfer coefficient, by appropriate routing of the flow of the small channels, for example in a zigzag shape.

[0024] A compact heat exchanger of the sort depicted in the figure may be manufactured advantageously of copper or copper alloy, stainless steel, aluminum, and other materials.

[0025] The exemplary embodiment of a heat exchanger according to the present invention described above may be used advantageously as an inner heat exchanger in a CO₂ air conditioner in vehicles, in particular in motor vehicles.

[0026] Here, an inner heat exchanger having the structure described above and the stated flow conditions may be designed for high pressures up to approximately 150 bar.

[0027] In this case, the first (high pressure) flow channel, marked in FIG. 1 by black arrows, lies in a first flow path from a gas cooler to an evaporator, and the second (low pressure) flow channel, marked by hatched arrows in the figure, lies in a second flow path from the evaporator to a compressor of the vehicle air conditioner.

[0028] In the first flow path a high pressure up to approximately 150 bar and a high temperature may prevail, and in the second flow path a low pressure up to approximately 60 bar and a relatively low temperature may prevail.

[0029] To those skilled in the relevant art it will have become clear on the basis of the above description that the heat exchanger depicted in FIG. 1 is merely schematic and exemplary, and that another geometry which differs from a lamellar shape of the heat transfer plates, such as a cylindrical structure, may be implemented.

[0030] In the embodiment according to FIG. 2, there are in first heat transfer plate 1, for example, two small channels 11 though which the coolant flows on the low pressure side. The approximately U-shaped cross section of small channels 11 is closed by second heat transfer plate 2, so that the coolant cannot escape. To attach the two heat transfer plates 1, 2 to each other, a connection 20 is provided, for example a soldered connection. Small channels 12 through which coolant flows on the high pressure side are located exactly above each of small channels 11 of the low pressure side, but on the side of second heat transfer plate 2 which faces away from first heat transfer plate 1. The long side of small channels 12 of the high pressure side might be closed by an additional heat transfer plate 3 which is not shown in FIG. 2. The coolant flows through small channels 11, 12 according to the counterflow principle.

[0031] The heat exchanger in FIG. 2 may be made even more compact according to the arrangement in FIG. 3, by positioning the orifices of small channels 11 of the low pressure side at an offset from the orifices of small channels 12 with the flow from the high pressure side. A first bridge 22 of first heat transfer plate 1 which lies between the two orifices of small channels 11 is now exactly opposite the orifice of a small channel 12 with the flow from the high pressure side, in such a way that it absorbs the forces produced by the pressure difference in channels 11, 12. Because small channels 11 on the low pressure side are offset from small channels 12 on the high pressure side in tiers with different pressure levels, it is possible to reduce the requisite thickness of heat transfer plates 1, 2. This is achieved by having more of the forces produced by the pressure difference in channels 11, 12 absorbed by bridge 22 between the orifices. This measure makes it possible to reduce the volume and in particular the mass of the heat exchanger significantly. This is especially important for materials of great density which also have great strength. Through this mass-reducing measure it is now possible also to use materials of great density, since the mass of the heat exchanger is no longer determined only by the density of the material, but also by the density of the fluid which is in small channels 11, 12. In particular, materials having great strength such as stainless steel or copper may be used.

[0032] In the embodiment according to FIG. 4, second bridge 24 is reduced in comparison to first bridge 22 of the embodiment according to FIG. 3, to the point where the stresses caused by the pressure differences in the bridges are of exactly such a magnitude that the permissible stresses of the particular material are not exceeded. The orifices of small channels 11 are again positioned at an offset from the orifices of small channels 12. The embodiment according to FIG. 4 makes it possible to design the heat exchanger even more compactly.

[0033] The thickness of heat transfer plates 1, 2 might vary in the range between 600 and 1000 μm, the dimensions of small channels 11, 12 between 400 and 1400 μm, and the width of bridges 22, 24 between 350 and 800 μm, with a pressure differential εp of up to 225 bar and copper as the material. The dimensions may vary upward or downward as appropriate, however, and in any case do not constitute a restriction. 

What is claimed is:
 1. A heat exchanger having a first channel through which a stream of refrigerant flows on the high pressure side and a second channel, separate from the first channel, through which refrigerant flows on the low pressure side, wherein the first and second channels each have a large number of small channels (11, 12, 13, . . . ) formed in or on individual heat transfer plates (1, 2, 3, . . . ), and a plurality of layers of the heat transfer plates are joined together.
 2. The heat exchanger as recited in claim 1, wherein the small channels conduct the flow in such a way that the refrigerant stream on the high pressure side and the refrigerant stream on the low pressure side flow through the heat exchanger according to the counterflow principle.
 3. The heat exchanger as recited in claim 1 or 2, wherein the heat transfer plates are of a lamellar shape.
 4. The heat exchanger as recited in one of the preceding claims, wherein the refrigerant on the high pressure and low pressure sides is CO₂.
 5. The heat exchanger as recited in one of claims 1 through 4, wherein the hydraulic diameter of the small channels (11, 12, 13, . . . ) is chosen such that the product of the heat transfer coefficient and the heat transferring area on the high pressure side corresponds to the product of the heat transfer coefficient and the heat transferring area on the low pressure side.
 6. The heat exchanger as recited in one of the preceding claims, wherein the manner in which the small channels conduct the flow is chosen such that the product of the heat transfer coefficient and the heat transferring area on the high pressure side corresponds to the product of the heat transfer coefficient and the heat transferring area on the low pressure side.
 7. The heat exchanger as recited in claim 6, wherein the small channels are routed in a zigzag pattern in or on the heat transfer plates.
 8. The heat exchanger as recited in one of the preceding claims, wherein the material of the heat transfer plates (1, 2, 3) is chosen from a group which includes copper and copper alloy, stainless steel, aluminum, and additional materials.
 9. The heat exchanger as recited in one of the preceding claims, wherein the small channels are produced in or on the heat transfer plates using a manufacturing process that removes or builds up material.
 10. The heat exchanger as recited in one of the preceding claims, wherein the lamellar heat transfer plates are enclosed between two opposing cover plates (8, 9), of which the first cover plate (8) has inlet and outlet orifices (14, 15, 16, 17) in each case for refrigerant on both the high pressure and low pressure sides.
 11. Use of the heat exchanger as recited in one of the preceding claims as an inner heat exchanger in a CO₂ air conditioner in vehicles, in particular in motor vehicles.
 12. The use as recited in claim 11, wherein the inner heat exchanger is designed for high pressures of the CO₂ refrigerant up to approximately 150 bar.
 13. The use as recited in claim 11 or 12, wherein CO₂ flows through the first channel of the inner heat exchanger in a first flow path from a gas cooler to an evaporator and through the second channel in a second flow path from the evaporator to a compressor of the vehicle air conditioner.
 14. The use as recited in one of claims 11 through 13, wherein in the first flow path a high pressure up to approximately 150 bar and a high temperature prevail, and in the second flow path a low pressure up to approximately 60 bar and a lower temperature prevail.
 15. The heat exchanger as recited in one of the preceding claims, wherein the small channels (11) of the first heat transfer plate (1) are positioned at an offset from the small channels (12) of the second heat transfer plate (2). 